The poem is about the lack of freedom. The monstrous thing is that our numbered citizen doesn't seem to be particularly concerned that he has any. He did what he was told by the authorities; he was a good little boy for the nanny government.

"The Unknown Citizen," a poem by W. H. Auden, deals significantly with issues of freedom. For example, the citizen is valued mainly because he serves the community, not because he freely pursues his own independence. He complies with the expectations of just about every organization of which he is a part. He seems to feel no deep-seated hunger for personal liberty. He echoes popular opinion (rather than thinking for himself in any fundamental way). As the final lines imply, he never seems to have given much thought to the idea of freedom.

"The Unknown Citizen," a poem by W. H. Auden, deals
significantly with issues of freedom. For example, the
citizen is valued mainly because he serves the community, not
because he freely pursues his own independence. He complies with
the expectations of just about every organization of which he is a
part. He seems to feel no deep-seated hunger for personal
liberty. He echoes popular opinion (rather than thinking for
himself in any fundamental way). As the final lines imply, he never
seems to have given much thought to the idea of freedom.

The tone of the poem is burlesque and sarcastic, so is it safe
to asume that in a way W. H. Auden is actually saying that
conformity destroys freedom and we should fight conformity?